1. Field of the Invention
The present invention relates to a manufacturing method of an application roller that applies a coating solution onto a surface of a substrate and also relates to an application roller.
2. Description of Related Art
Electrodes (positive electrode and negative electrode) of a lithium ion secondary battery are each prepared by forming a thin film on the surface of the substrate such as aluminum foil or copper foil as a current collector. The thin film is formed by applying a paste including an active material and a binder resin onto the surface of the substrate and by drying the paste. Increasing drying rate of the paste is effective in reducing costs for equipment for performing such an application operation. However, inflow of hot air for drying the paste is likely to cause migration in which the binder resin is liberated from the active material in the applied paste and segregated in a depth direction of the applied paste. As such, it is a common practice to adopt a two-step coating process which includes first applying the binder resin onto the surface of the substrate and then overlaying a layer of a binder-free composition. The binder resin is necessary for binding an active material to the other active material and binding the active material to the substrate. When used in excess, however, the binder resin may interfere with the migration of lithium ions, impairing battery characteristics. In the light of the battery characteristics, therefore, high-precision control of the thickness of film which is formed by applying the binder resin solution onto the substrate is important.
However, the coating solution is normally applied to the substrate at a local area thereof and hence, an application roller need be masked in correspondence to an area not to be coated. Japanese Patent Application Publication No. 5-15824 (JP 5-15824 A) describes a method and apparatus for locally applying the coating solution with a masked application roller. According to JP 5-15824 A, an upper application roller forming a roll coater has a masking film of a predetermined width wrapped therearound. A lower feed roller of the roll coater is rotated so as to pass a substrate sheet through space between the upper and lower rollers while the coating solution is supplied to the space. Thus, the surface of the substrate sheet is locally coated with the coating solution except for an area corresponding to the portion of the upper application roller that is covered with the masking film. Such a technique is advantageous in that a desired coating application is accomplished simply by wrapping the easily available masking film around an outer periphery of the roller and that the application roller is easily adapted for the specifications of product to be coated by changing the shape of the masking film.
However, the coating technique described in JP 5-15824 A has the following problems. Firstly, the masking film, which has a given thickness, forms a step between an end of the masking film and the outer periphery of the application roller when wrapped around the application roller. The masking film must be reduced in thickness in order to reduce the step height. However, since the masking film is in friction contact with the outer periphery of the application roller being rotated, the masking film is prone to failure such as fractures or wrinkles if the masking film is made too thin. That is, the masking film is required of a given thickness (at least 10 to 20 μm). On the other hand, as shown in FIG. 7, a phenomenon occurs in which a coating solution P is drawn by surface tension to an edge F1 of a masking film F having the given thickness. In the neighborhood of the edge F1 of the masking film F, therefore, some coating solution is drawn by surface tension to be added to the coating solution retained by an application groove 101 so that an application roller 100 retains a larger amount of coating solution P near the edge of the masking film F than at a central portion thereof. This leads to a problem that a coating film W formed by applying the coating solution P onto a substrate K has a greater thickness H1 near an edge WE thereof than a thickness H2 at a central portion WC thereof, as shown in FIG. 8. According to the results of experiments made by the inventors, an increased edge thickness (H1-H2), that is, an increased amount of edge thickness near the edge WE relative to thickness at the central portion WC, was in the range of 200 to 300 nm, as shown in FIG. 9, in a case where masking films F having thicknesses in the range of 10 to 20 μm were used. The increased edge thickness accounts for 5 to 8% of the thickness of the coating film, having a significant influence on battery performance. Hence, there exists a demand for an application roller and a manufacturing method of the application roller that are adapted to reduce the increased edge thickness of the coating film as much as possible.
According to the coating technique disclosed in JP 5-15824 A, the masking film is simply wrapped around the application roller and hence, the position of the masking film relative to the application roller is unstable. This results in another problem that the coating film formed by applying the coating solution onto the substrate is varied in its edge position.